Low value high aromatic middle distillate range streams from any cracker units such as Light Cycle Oil (LCO) stream of Fluid catalytic cracking (FCC) units, because of high Sulphur, Nitrogen and Aromatic content; are very low value streams. Historically, these streams were being used as blend stocks in middle distillate and as cutter stock in fuel oil. Because of growing environmental consciousness the emission norms are getting stringent and at the same time the demand for high quality ultra low sulphur diesel (ULSD) is increasing rapidly. Therefore, these streams can no longer be used directly as diesel pool blending stocks. Further, the worldwide demand of Fuel oil is also declining sharply hence another conventional usage of these types of low value high aromatic streams as cutter stock has also become significantly low. Presently, in most of the refineries, these streams are being blended and hydrotreated along with diesel hydrotreater (DHDT) feed. In some refineries, these are also hydrocracked along with Vacuum Gas Oil (VGO) in a high pressure hydrocracker. But hydrotreating these streams for Diesel pool blending stock is inefficient means of utilizing costly hydrogen since hydrotreating only removes sulphur but cetane number remains much below the limit of Euro-III/IV/V diesel pool Cetane specification. Hydrocracking these streams along with VGO in conventional high pressure hydrocrackers improve diesel yield, but at the same time also increases naphtha generation substantially. Moreover, processing these streams in VGO hydrocracker also reduce throughput of VGO. Consequently, with both these options actual potential of high aromatics middle distillate range streams remains underutilized.
The middle distillate range streams of cracker units such as LCO stream of FCC units are rich in aromatic compounds. Depending upon the severity of the operating conditions the total aromatics in these types of streams vary from 50 to 90 wt %, in which mono-aromatic compounds are only 20-30 wt %, whereas di-aromatics are 50-70 wt % and the rest 5-10 wt % are polycyclic aromatic type of compounds. However, the poly-aromatic types of compounds present in these types of streams rarely have more than 3 rings.
The middle distillate range streams boiling between 150 and 400° C. are often considered as low value streams owing to their high aromaticity and high sulfur and nitrogen levels, since these streams are difficult to accommodate in diesel pool. Examples of such streams are light cycle oil (LCO) from FCC and Coker Gas Oil from Delayed Coker units. These low value streams in refineries have very low cetane values, thus accommodating them in diesel pool requires higher consumptions of costly hydrogen. Merely hydrotreating them to improve their qualities is very inefficient means of utilizing hydrogen. The judicious use of hydrotreating, hydrocracking with ring opening functionality in staged manner is required. For example, only hydrotreating these streams may result in diesel range product with poor cetane and hydrotreating with conventional hydrocracking in existing hydrocrackers may result in high yield losses in terms of lighter products of light naphtha and heavy naphtha of inferior quality with medium quality diesel range product. Further, these streams constitute high levels of aromatics which are valuable in some fields of art. If they are converted to utilizable compounds, they can be very good feed stock for petrochemical products. Therefore, it is preferable to convert these aromatics in more valuable and usable compounds and use rest of the portion rich in naphthenes and paraffins as diesel can be a viable and economical option for refiners.
Different techniques of conversion of highly aromatic, low value middle distillate streams of cracker units into high value, high octane motor gasoline, and USLD have been described in many patents, such as U.S. Pat. No. 8,404,103 described the technique of conversion of highly aromatic feeds such as LCO into low sulphur diesel and high octane naphtha using mild hydrotreating and hydrocracking reactions. In this patent it has been claimed that the octane quality of the gasoline or naphtha fraction can be improved by allowing slippage of organic nitrogen to hydrocracker reactor by optimizing the operating condition of the hydrotreater. The patent also claims to obtain naphtha having at least 50 wt % mono aromatics concentration with RON value at least 85 and diesel with sulphur concentration less than 10 ppmw.
The U.S. Pat. No. 8,066,867 claims to develop process for converting LCO into low sulphur diesel and high octane naphtha using mild hydrotreating and hydrocracking reaction. The patent also claims to produce naphtha with RON value at least 85 units and diesel with sulphur content less than 10 wt-ppm.
The U.S. Pat. No. 7,837,860 describes an integrated process for hydrodesulfurization of middle distillate streams and hydrocracking of highly aromatic hydrocarbon streams such as LCO for production of low sulphur diesel and high octane naphtha.
The U.S. Pat. No. 7,384,542 describes about the invention of process for production of low sulphur diesel and high octane naphtha form highly aromatic and substantially dealkylated streams like LCO. The process claims to produce mono-cyclic aromatic compounds having boiling points in the range of naphtha.
The U.S. Pat. No. 4,990,239 of Mobil Oil Corporation describes their invention for production of Gasoline and distillate fuels from Light Cycle Oil. The patent claims to produce Gasoline with RON value at least 87 units and low sulphur diesel with cetane value at least 30. Another patent, U.S. Pat. No. 4,985,134 from the same assignee claims to produce gasoline and diesel fuel of cetane number at least 35 from LCO.
U.S. Pat. No. 4,828,676 discloses a process for production of ultra high octane gasoline from Aromatic distillates using hydrocracking reaction. In their process, they have disclosed to use iron as one of the components in the catalyst for ring opening purpose.
Some patents are also available where highly aromatics and dealkylated streams like LCO has been converted into Petrochemical feed stocks and xylenes. The U.S. Pat. No. 7,368,619 discussed about the process where diesel and aromatic compounds including xylene have been produced from LCO. The U.S. Pat. No. 7,271,303 claims to develop a process for producing diesel and aromatic compounds integrating hydrocracking and catalytic reforming. Another patent U.S. Pat. No. 7,265,252 discloses production of Xylene from LCO by selective hydrocracking reaction.
The common shortcoming in all of the above discussed inventions is the property of ULSD which is produced as one of the streams during the process. Although diesel produced in the process is low in sulphur but the other properties such as Cetane Number remains much lower than that required for EURO-III/IV/V diesel pool. Therefore, for complete upgradation of these types of high aromatic middle distillate range cracker streams, the cetane of the ULSD produced during the process also needs to be improved.
The U.S. Pat. No. 4,723,963 of Exxon Research and Engineering Company discussed about invention on selective oxidation of aromatic compounds for improve of cetane rating of middle distillate. The patent claims that oxidation of aromatics particularly alkylaromatics and hydroaromatics at benzylic position improves the cetane rating of the fuel. The patent has also disclosed the cetane rating of different aromatic compounds normally present in middle distillate range fuels and their corresponding oxidized compounds.
U.S. Pat. No. 7,501,054 and U.S. Pat. No. 7,501,374 discuss about the invention of process and catalyst for upgrading diesel fuel by introducing oxygen. The patent also discloses a process to convert alkyl-naphthene-aromatics compound to alkyl ketones.
It is therefore, important to develop an integrated process of interest that can upgrade these types of high aromatic content middle distillate range streams of cracker units into value added products keeping view that all potential are utilized to their limits.